The storage of radioactive elements must meet very severe ongoing safety and reliability criteria. In particular, protection in relation to the exterior environment must remain effective for several tens of years, or indeed several centuries. Radioactive waste is ranked according to several levels. The most sensitive radioactive materials, that is to say those which exhibit the highest radioactivity, are stored in amorphous glass which is a neutral material from the radioactive standpoint, thus forming a barrier to the propagation of radioactivity. In practice, radioactive waste is embedded in glass by high-temperature fusion, whereby blocks of glasses are created. The radioactivity is then held captive in these blocks of glasses which are generally in the form of tubes to facilitate storage.
On a scale of a few years, or indeed several tens of years, it is known that protection against radioactive leaks remains effective. However, beyond this observed duration, there is no certainty about the absolute effectiveness of glass against leaks. In particular, the radioactive atoms held inside the glass could have a non-negligible impact over time, possibly eventually causing radioactivity leaks.
A characterization of the structure of amorphous glasses is therefore necessary in order to anticipate possible long-term problems. In particular, it is necessary to characterize the influence of radioactive elements on the structure of the glass, so as to ascertain notably whether radioactive radiation modifies this structure, how or according to what law, thus making it possible to ascertain whether protection is maintained over the long term or whether it weakens, to what extent and how to remedy this.
Unlike crystalline matrices, amorphous matrices are devoid of any periodic structure. The characterization of such structures is therefore a problem of great complexity, where modeling plays a significant role. Therefore, this characterization relies rather on obtaining information in the small interatomic distance region. Experimentally, a set of diagnostics may be implemented, which include nuclear magnetic resonance (NMR) or Wide Angle X-ray Scattering (WAXS).
In order to study the disordered structure of an amorphous glass, it is possible to use the statistical approach consisting in obtaining, on the basis of spectra recorded experimentally by the WAXS method, information about the atomic distribution, which is one of the most characteristic representations of an amorphous structure.
In this context, a significant quantity is the elastic scattering, coherent, dependent or interfering, inside the glass on the basis of an emitted X-ray and containing information about the constructive interferences which occur when the electromagnetic wave passes in proximity to the atoms which are viewed as scattering centers. X-ray diffraction is a coherent and elastic scattering phenomenon which occurs when X-rays interact with matter. The diffracted wave results from the interference of the waves scattered by each atom.
An experimental spectrum which is recorded by the WAXS method is recorded over the widest possible region of scattering angles. In this case, it is the resultant of elastic and inelastic scattering phenomena, which are dependent for small scattering angles and quasi independent for large scattering angles. It is therefore necessary to extract just the fraction of dependent coherent signal by correcting the initial spectrum for the various phenomena which alter it. This requires notably a knowledge of the scattering of the incident beam by the residual gas present around the specimen studied, of the absorption by this specimen and of the various polarizations which occur when the X-ray beam is reflected at the surface of the specimen or of the crystal of the monochromator.
These various corrections are related to the specifics of the diffractometers used, in particular to the type of monochromator, to the nature of the residual gas surrounding the diffractometer used, to the type of detector, to the presence of filters in the path of the X-rays and to the scattering of the beam by reflection or by transmission. The other corrections applied to the experimental spectrum which may not be estimated experimentally like the independent coherent scattering or the independent incoherent scattering, are evaluated in a theoretical manner with the aid of tables arising from ab-initio calculations.
The application of the various corrections makes it possible to construct the structure factor of the glass, and then the radial distribution function. It makes it possible essentially to quantify the interatomic distances, as well as the coordinance numbers of the matrix studied.
All the operations described above, as well as the calculation of the radial distribution function, must be performed by successive steps:                on the one hand, the obtaining of an appropriate structure factor requires several iterations in the course of which corrective parameters may be adjusted;        on the other hand, the calculation of the radial distribution function by Fourier transform comes up against the effect of spectrum truncation in the region of the high values of the modulus of the scattering vector, introducing mathematical artifacts that are difficult to discern subsequently.        